Nevertheless, whether the present Universe is open, closed or flat is still being debated, despite the ever-stronger support for a Flat Universe. The key factor is the mass density of the Universe. If this amount lies below a precise value, then gravitational forces will be insufficient to finally halt the expansion that would eventually result in all matter throughout space being pulled back into closure; in the open case, expansion is forever in (apparently) the one and only Universe. As of now, inventories of mass within the Universe have come up way short of the amount actually identified as to their nature needed to maintain a closed Universe but with further observation and experimentation the gap is narrowing. If the interpretation of WMAP data is essentially "on target", then the condition that the total Universe mass is close to the Critical Mass allows for a fairly precise estimate of how much mass must be found and accounted for.
The bulk of the missing mass (and energy particles that have mass) is believed to exist as Dark (non-luminous, i.e., does not give off [detectable] electromagnetic radiation) Matter of still uncertain types and/or as neutrinos, and thus remains at present "invisible" to astronomers' detectors. Theory presently holds that most proposed kinds of Dark Matter do not interact with baryonic (Ordinary) Matter, although some Dark Matter may be baryonic. Dark Matter is currently difficult to detect and thus its amount is hard to quantify. Dark Energy, a closely related topic, is mentioned briefly in this subsection but will be examined in some detail on the next page. Suffice for now to state that the main role of Dark Energy seems to be that it has some property the counteracts the effect of gravity in slowing down or decelerating the Universe's expansion. Dark Energy thus produces an accelerating force which is the best current explanation for the recent observation that the Universe is now expanding or speeding uup(and appears to have resumed a net expansion in the last few billion years).
Some of the Dark Matter seems to associate with the galaxies. Theory says that Dark Matter plays a key role in galactic development and stability. The galaxies themselves contain insufficient mass to provide the gravitational strength needed to hold them together. Since they are intact, it is a logical inference to assume the existence of Dark Matter which supplies the gravitational forces that maintain them. A computer simulation reported in late 2003 indicates the bulk of Dark Matter occurs in large "clumps" distributed throughout intergalactic space; see further discussion below. Black holes may serve as collectors or maintainers of this Dark Matter.
A recent estimate states outright that Dark Matter and Dark Energy together comprise ~95% of the Universe's mass (remember Einstein's mass-energy equivalency), with varieties of normal or baryonic matter accounting for the remainder are thus less than 5%. This means that most of the matter in the Universe presently is invisible to detection from Earth (the billions of luminous galaxies, each with billions of stars therefore make up only a tiny fraction of the Universe's total mass).
This humongous amount of Dark Matter is postulated by inferences drawn from observed effects; indirect evidence comes from the behavior of galaxies and galactic clusters which seem to need this superabundance of mass to account for their stabilities and motions. For example, the velocities of stars in outer galactic spiral arms is much greater than predicted from the Newtonian 1/r2 force law, implying excess external mass. Thus a sheath of invisible Dark Matter/Energy acts to prevent galaxies of spinning apart by holding the fast-moving outer stars in a gravitational bind.
Another line of evidence comes from the gravitational lens effect (see text and illustrations in the Preface page of this Section) - much more mass than observable is needed to account for the degree of bending of space as predicted by General Relativity. This is revealed by a greater curving of light from more distant light sources (therefore displaying larger displacements than expected) than would be caused only by the mass of the specific galactic cluster whose gravitational influence is being tested. Another sign of concentrations of hidden mass relates to directional movement of galaxies near enough to observe and measure this motion. Close to home, our Local Group (including the Milky Way) of galaxies is moving through space in the direction of the Constellation Centaurus at greater than expected velocities, under the influence of an invisible mass concentration dubbed "The Great Attractor."(by itself, the Milky Way is moving at about 2.1 million km/hr towards the region around the Constellation Leo). Also, huge masses of glowing gas whose molecules are rapidly moving and hence indicate very high temperatures have been detected; being very hot, they should fly apart but clearly are holding together, indicating the attractive action of great quantities of invisible mass.
There is evidence that older, more primitive stars (that have a paucity of those heavier elements that were produced in stars and dispersed by supernova explosions) contain around them higher concentrations of Dark Matter. But, much (most?) of the missing mass may be tied up in Black Holes; billions probably exist throughout the Universe, and many, if not most, of the galaxies have Black Holes in their central cores. A Black Hole with immense mass has been verified at the center of the Milky Way, around which the inner stars revolve about the Hole at speeds up to 3 million km/hr as they spiral inward to eventually be sucked in (by comparison, the Sun orbits around the galactic center at ~790,000 km/hr and the Earth around the Sun at ~108,000 km/hr).
The composition of Dark Matter is still speculative. Candidates are shown in these diagrams, modified from that which appears in the page dealing with the nature of the Dark Universe found on the University of Oregon Astronomy site we have referenced several times in this Section:
Some of the baryonic Dark Matter occurs in what is called MACHOs (for MAssively Compact Halo Objects), consisting of baryons (protons, neutrons) and other matter (probably some fraction of the neutrinos pervading space) in the non-radiating dark halos now known to distribute around galaxies and in intergalactic space (see below). In the halos they constitute most of the faster moving Hot Dark Matter (HDM). MACHOs contain enough extra mass to provide the gravitational boost that holds galaxies together (motion in the spiral disk would otherwise cause a galaxy to fly apart). Dwarf galaxies and Wwhite and Black Dwarf stellar remnants - too small for ready detection in more distant space - may also abound in this material. Black Holes, Neutron stars, and Dwarfs, undetectable by visual means but identified by their gravitational effects on nearby visible stars, also make major contributions. In fact, the same (as yet undetected) material as occurs in MACHO's may also make up planet-sized Black Holes, whose numbers in the Universe can be huge; this is not yet verifiable if a B.H. is "standing alone", i.e., does not have a companion star(s) feeding it material that becomes excited and luminous.
One MACHO type is the neutrino whose existence has now been proved. Once thought to be massless, a tiny but real neutrino mass has only recently been verified - a determination that could account for much (most?) of the "missing" Universe mass. Neutrinos (electrically neutral) are found not only in haloes but appear to pervade the "empty space" of the Universe. An announcement in June 1998 by a Japanese-American research team may dramatically change the role of neutrinos in the mass balance sheet. Using a deeply buried detector and a huge array of detectors, they have been able to capture light signals set off by neutrinos that disclose these extremely small particles to have an infinitesimally minute mass, about one ten-millionth that of the electron. But, because of the extremely large population of neutrinos throughout the Universe (like the Cosmic Background Radiation, they too are a residue of the Big Bang, being especially produced in abundance during the first few minutes when protons were being fused into Deuterium and Helium nucleii releasing energy as neutrinos), the cumulative mass of these particles may make a sizeable contribution to the inventory of missing mass. But other theoretic MACHO(s) that remain to be discovered (either from future particle accelerator experiments or from still-to-be-built, more sensitive space detectors) will likely constitute the bulk of the MACHO population.
Concerning neutrinos as a component of Dark Matter, until recently there was still considerable uncertainty as to whether neutrinos (there are several varieties) have any mass at all. They rarely interact with matter and are thus very hard to detect. At this instant, billions of neutrinos are passing through your body, and perhaps one or two at most will meet with an atom. Experiments in several countries are attempting to find out more aboutthe elusive neutrino. Below is one example of an experimental detection setup. Several thousand feet below the surface, in a large alcove in a nickel mine near Sudbury, Ontario (the overlying rock screens out most other high energy particles that might produce false signals), is a 12 meter diameter sphere containing heavy water (deuterium instead of Hydrogen). Around it are a bevy of event detectors. A few events have been ascertained, but the number of solar neutrinos appears lower than theory had predicted.
If the neutrinos are finally assigned a specific (but tiny) mass, they must be a major component of Dark Matter. The estimated ratio of Dark Matter neutrinos to Ordinary Matter neutrinos is 100,000,000 to 1.
A significant fraction (perhaps the bulk) of the missing matter, however, seems to occur in Cold Dark Matter (CDM), probably in the form of slow-moving WIMPs (Weakly Interacting Massive Particles) - elusive matter/energy that does not interact with electromagnetic (EM) or strong nuclear forces. These appear to be heavy nonbaryonic particles (50 to 100 times the mass of a proton) of still unknown nature. They may also include two specific particles that have been postulated but not yet found experimentally: the neutralinos and the axions - both predicted to exist according to SuperSymmetry theory. Several high energy accelerators are either now operational or are being built (the best and biggest is CERN's Large Hadron Collider [LHC] in Switzerland) to find direct evidence of the existence of WIMPs or some similar form of matter.
Recent reports from several investigator teams claim that much of the Dark Matter is accounted for by the very hot gases that are believed to pervade the intergalactic space that appears "empty" compared with galaxies and nebular clusters of visible gases. A clue to the presence of these gases (undetected at Visible and longer wavelengths) was found in UV spectrum data but the signal was weak. However, when Chandra data for X-ray radiation was collected from intergalactic regions, indications were for very hot gases in signficant concentrations in the so-called void. An estimate of gas densities was obtained by calculating the weakening of certain wavelengths as radiation from a gas-rich region passes through a nebular mass. This is shown diagramatically:
The relative amounts of Dark Matter and Dark Energy have been calculated based on data collected in the past few years. This is conveniently displayed in this pie-chart diagram, made in the mid-1990s, that includes all physical entities:
In this diagram, Ordinary (detectable) "Matter", mainly in the star-studded galaxies, amounts to only 0.5%. Photon radiation moving about the Universe contributes just 0.005%. The MACHOS make up the Ordinary (including nonluminous) Matter (4%) and WIMPS comprise the Exotic Dark Matter (25%). The bulk of the ingredients in the Universe is what is known as "Dark Energy", and accounts for ~70% of the estimated total mass/energy; conjectures as to its nature are described on the next page in the paragraphs that deal with the repulsive energy that could relate to Einstein's Cosmological Constant.
The WMAP data have resulted in a further refinement in the percentages of Dark Matter and Dark Energy. This diagram shows a shift to a 3% larger amount of Dark Energy with a 2% decrease in Dark Matter:
In February 2003, F. Nicastro and his associates at the Center for Astrophysics of Harvard's Smithsonian Astronomical Observatory reported a more precise calculation of the numbers for Ordinary Matter. Of the 4% that comprises the total amount, about 0.4% is luminous (in the visible range) as the stars making up galaxies. The remainder consists of H, He, and other Baryons that occupy both galactic halos and beyond in intergalactic space. These exist in a high temperature (105 to 107 degrees Kelvin) "fog" that they have detected using both FUSE (Far Ultraviolet Spectrometer Explorer) and Chandra (X-ray) data. This fog is left over from the galactic formation process and serves to provide the extra mass needed to gravitationally bind together galactic groups (they studied the Local Group around the Milky Way) in clusters.
The roles of Dark Matter and Dark Energy in the observed Universe are still being worked out. Both, or perhaps Dark Matter in particular, seem to be needed to keep the galaxies intact and to prevent them from collapsing into each other. It appears that the gravitational forces from the matter within a galaxy are insufficient to keep them from flying apart and dissipating into intergalactic space. Dark Matter should provide the mass needed to furnish the gravitational stability that maintains the integrity of the galaxies once formed. In fact, several models for its existence and nature consider the bulk of Dark Matter to occupy large invisible halos surrounding the galaxies, serving as the major gravitational "glue" that holds them together. And, as will be discussed on the next page, Dark Energy, as inferred by theorists, is a positive force that is responsible for the increasing expansion of the Universe (thus, it counteracts gravity)
An imaginative explanation for why Dark Matter (in terms of mass) has so far eluded scientists as to proving its existence and nature has been proffered by Dr. Jonathan Feng and his group at the Univesity of California-Irvine. They postulate this matter to be hidden particles that reside in an extra dimension beyond the three spatial ones that we sense. This is plausible if the multi-dimensional space described at the bottom of page 20-1 is a reality. This model may be provable by this means: the extra-dimensional mass particles would tend to collect around bodies that have strong gravitational pulls. Under this influence, the particles would tend to collide with each other more often, producing neutrinos with especially high energy. Experiments to test this hypothesis have been proposed.
One test for the existence of Dark Matter (and probably also Dark Energy) is that of the (Einstein) gravitational lensing effect (bending of light as it passes by a massive galaxy). If only the luminous matter in the galaxy, supplemented by assumed mass in contained black holes, is operative, the amount of deflection should be less than observed. In fact, that deflection is greater than this simpler case, and thus has been attributed to at least MACHOS and/or some amount of WIMPS that are emplaced in and around the galaxy. Using gravitational lensing techniques, the size of the Dark Matter distribution concentrated around galaxies has been estimated to be in a roughly spherical arrangement (the 'halo') up to about 5 times the radius of each galaxy examined so far.
Gravitational lensing has been used to create this image of a group of galaxies, in which the pinkish halo around each is the estimate of the size and position of Dark Matter holding each intact:
Incontrovertible proof that Dark Matter and Dark Energy really exist, and insights into their nature, rank near the top of priorities that astronomers and cosmologists intend to address in the first decade of the 21st Century. A number of approaches besides gravitational lensing have been proposed. One is to compare the size of a galaxy as seen in visible light to the size of its associated hot matter (the "fog") that gives off strong X-ray radiation. Look at this pair of images of a galaxy in the Virgo galactic cluster:
Another, oft-cited, example of the X-ray envelope is the NGC 2300 galaxy group, consisting of a number of individual galaxies as seen in Visble light that do not show up as individuals when X-radiation (measured by ROSAT) is assigned colors to indicate its extent.
How do these X-ray images indicate the presence (existence) of Dark Matter? The argument is that the galaxies would have flown apart by now but are held together by the mass associated with the Dark Matter that encloses them. Consult the University of Oregon page cited above for illustrations that define this process.
A variant to the size argument is to study the shapes of galaxies that have characteristics explainable by the presence of Dark Matter. MGC 720, some 80 million light years away, shows differences between Visible and X-ray images that can be postulated as imposed by Dark Matter in the halo surrounding the galaxy. As seen below, the galaxy has a flattened elliptical shape when viewed in Visible light but as imaged by the Chandra X-ray Telescope the core appears round and the surrounding excited matter is distributed spherically. A calculated model for this difference works well when a concentration of Dark Matter is introduced. There is a hypothesis that galaxies will attract greater amounts of Dark Matter surrounding them than would be found in intergalactic space.
In June, 2003 another report about the action of Dark Matter gave a strong indication of just how much mass is involved. Below is a Chandra view of the galaxy cluster Abell 2029, located about 1 billion light years from Earth. This X-ray image shows a central radiating mass (an elliptical supergalaxy that resulted from merger of multiple galaxies) and a huge cloud of glowing hot gas that is interpreted as under direct control by this Dark Matter, which is estimated to be equivalent to a hundred trillion times the mass of the Sun.
In July of 2003 another image was released which may actually show the Dark Matter as a faint glow (arbitrarily rendered by a blue tint) pervading a cluster of galaxies:
To obtain this image, the HST was trained for a total of 120 hour of light-collection time on the cluster CL0024 which presently is some 4.5 billion light years away. Spread through the galactic group is the uniform glow which in itself has not been identified as to nature. But its distribution is much as expected for Dark Matter based on theory. If this glow indeed is really a property of Dark Matter, then it produces some luminosity, i.e., emits EM radiation that is normally so weak as to be undetected in shorter exposures of visible wavelength observations from both space and Earth telescopes.
Another similar image of two galaxy clusters in HH47 (in the Vega constellation) was released in December 2005 by a Johns Hopkins group. These astronomers used an inventory of stars that bend light to determine that there must be a surplus of Dark Matter (shown in purple) that is needed to allow and maintain the clustering involved. Their interpretion of this matter is that it in the "strange" state wherein individual particles do not collide as they surround the galaxies. The implication is that Dark Matter itself has an uneven distribution in the Universe, tending to be more dense around galaxies and galaxy groups. Here is a defining image published in their report:
Another line of evidence for the presence and role of Dark Matter is associated with NGC1404, the Formax galaxy cluster, shown below. The Chandra X-ray image shows galactic gases in blue, heated to as high as 10 million degrees Celsius. The gas clouds are moving at high speeds toward an (invisible) center. The presumption of a large, unseen mass of Dark Matter around the cluster gives the best (theoretically) explanation of the behavior of what has been detected. This cluster may lie at the intersection of two filaments of stretched out Dark Matter; that speculation is in line with the ideas of a network of filaments in the early Universe described on page 20-2.
Recently, studies of Dark Matter in the Ultraviolet suggest that one of its constituents appears to be ionized Hydrogen which does not produce a detectable signal from the myriads of Hydrogen clouds now known to populate galactic haloes and intergalactic space. But, chemical processes in these clouds produce ionized Oxygen, uncombined with the invisible ionized Hydrogen, which can be detected. This form of Oxygen is widespread in much of space beyond the galaxies and, together, with the ionized Hydrogen may account for a significant fraction of the daark matter.
Another potential approach to studying Dark Matter, including an attempt to prove its existence as commonplace within the Universe (rather than just a fleeting existence during collider experiments on Earth), is now underway with the installation of a 2.8 meter telescope, called the South Pole Telescope, located close to that point in the Southern Hemisphere. It will be capable of measuring small changes in conditions (primarily temperatures) around galaxy clusters; these changes would verify expected variations caused by Dark Matter/Energy as predicted by models now being developed.
From the above discussion, one might ask "Where is most of the Dark Matter?". Since it has yet to be found directly, the answer may lie in these possibilities: 1) in Black Holes; 2) in non-luminous galaxies (evidence for a "Dark Galaxy" has recently been reported); 3) present but "invisible" within galaxies; 4) in roughly spherical "halos" around each galaxy; 5) dispersed throughout intergalactic space.
An interesting alternative to Dark Matter as the stabilizing force is that of a concept called MOND, for Modified Newtonian Dynamics. This is reviewed by its originator, Dr. Mordecai Milgrom of the Weizmann Institute in Israel, in his article "Does Dark Matter Really Exist?" in the August 2002 issue of Scientific American or the October 2003 issue of Discover Magazine. Check this to be familiarized with its essentials. His ideas are derived from observations that such spacecraft as Pioneer 10, now 9 billion miles from Earth, are slowing down (the expectation would be for increasing velocities as the Sun's gravity weakens with distance). This would seem to imply that solar gravity is actually increasing as the distance lengthens.
In a nutshell, Milgrom postulates a constant of acceleration, called a0, that determines the gravitational behavior of matter. Acceleration values larger than a0 are consistent with the Newtonian Second Law in which F (force) = m (mass) times a (acceleration such as we observe experimentally for gravitational processes or for imposed accelerations [e.g., a rocket launch]). (Recall in the Preface that Newton's Second Law works well on Earth and in the Solar System but is not accurate when galactic scales are involved.) But in his proposal values smaller than a0 modify the behavior of galaxies, including the rates at which they rotate, in a way that obviates the need for Dark Matter's contribution to galactic gravity. Using this non-Newtonian approach, Milgrom's mathematical analysis of galaxy dynamics based on values of a's that are less than a0 reproduces most of the observed effects in the orbital patterns and rates of motion of stars within galaxies. He has tentatively determined that a0 decreases the Newtonian proportionality by one 10-billionth of a meter per second per second. This, rather than Dark Matter, causes stars in a galaxy to move faster than expected. Milgrom's model, initially disregarded by most cosmologists, is now attracting serious attention.
An excellent, clearly written article by Martin Rees and Priyamvada Nataranjan, entitled A Field Guide to the Invisible Universe appears in the December, 2003 issue of Discover Magazine
As this page has been intermittently added to with new information, we must close our discussion of Dark Matter with two more ideas. One simply contends that "some" of the Dark Matter is Ordinary Matter lying beyond the limits of the observed Universe. This matter then has its own gravitational influence on that Universe as though it were actually within the observed part. Interesting but not readily testable.
The second new idea relates to the concepts of Higgs fields and particles (Higgs Bosons) mentioned mainly in footnote 4 on page 20-1. Drawing upon interrelations between particles as treated by the SuperSymmetry model and Higgs fields, theory states that there may exist something called the LSP (for lightest superpartner). The LSP is the decay end product of a series of superpartners (those that can be postulated as complementary to ordinary Standard Model particles) created in the early stages of the Universe. The LSP contains Higgs-induced mass. If real, LSPs can account for much of the Dark Matter.
In August of 2006 a claim was made that direct evidence for action by Dark Matter had been found. Astronomers from the University of Arizona and elsewhere, examining the Bullet galaxy cluster using Chandra and other telescopes, were able to determine the mass within the cluster. They observed two kinds of intergalactic material of gaseous/particulate nature that had separated within the cluster. Thus:
Their interpretation assumed that within the cluster a smaller galaxy had passed through a larger galaxy (the stars are normally so far apart that few collisions occurred). The larger galaxy should have stripped off matter from the smaller, leading to some segregation. Most of that matter ought to reside in the interstellar gas that provides the bulk of the gravitational forces within the cluster. But they argue that this degree of gravity would be insufficiently strong to keep the galaxies intact during the process. They hypothesized that Dark Matter was needed to preserve the separated gases. If indeed Dark Matter was present in the amount they calculated, then it would bend starlight to a specific extent in the manner which allowed Einstein to prove Relativity. This was observed in about that level. The argument then concluded that Dark Matter rather than the gas was the factor that led to this set of observations. While the proposal is tantallizing, as usual other astronomers have come up with objections and alternate explanations. One such alternative considers gravity to vary in strength over cosmic distances.
An even better example of direct imaging of dark matter is this view of two colliding galaxies (official identity: MAGSJ0025). The blue denotes dark matter, as imaged by HST; the red is a Chandra X-ray image of gases:
The importance of the Dark Matter concept is that it appears to provide just enough total matter to provide the gravity that holds the Universe together. Those who favor the "Goldilocks Hypothesis" for the Universe's formation - not too hot, not too cold, just right" - argue that too much matter (and/or too little velocity of expansion) would have led to gravitational pull strong enough to counteract the expansion of the Universe, causing it to collapse on itself; too little matter (too much velocity) would have prevented matter from organizing into stars and galaxies. Such results would have been a Universe that did exist but never attained the right conditions for the appearance of biological beings and ultimately humans (and conceivably intelligence elsewhere; next page).
Dark Matter seems essential to the formation of stars and galaxies as well. There is insufficient Ordinary Matter to organize and maintain galaxies. Pockets of Dark Matter - whose distribution appears to be related to the ripples or inhomogeneities found by COBE and WMAP - were needed to assist in bringing clots of matter together to start galaxies. Today's galaxies seem held together by higher densities of Dark Matter that comprises the halo around both spiral and elliptical types; this matter also influences the rotational velocities within a spiral galaxy, such that its outer parts move at about the same rates as the inner parts.
A painstaking study by an international group of astronomers (NASA, ESA, CalTech) has produced the first 3-D depiction of Dark Matter in a selected region of the Universe. Using Hubble Space Telescope data, they detected this matter indirectly, through its effect on passing light. This yields information on concentrations of Dark Matter at various distances from Earth, using the standard candle approach to determining just how far away is each clump of Dark Matter. In this way they could arrive at how the matter was distributed at different times of Universe history. These two diagrams show aspects of their study:
Now, on to Dark Energy per se, which comprises the bulk of the physical entities that make up the Universe.